With a bottom grate, I can really put the wood to it and it burns fairly clean as it has all the air it needs and the refractory pit really concentrates the heat. This means I have about three to four hours to bring the storage up to temp. If there is no heating demand the damper shuts and my dump zone rapidly comes on. It will keep heating the storage until a thermoswitch hits a high setpoint and then the three way valve switch back to recirculating. One the temps up a bit I flip a switch and the three way valve switched to the coil in the storage. If there is demand from the zones it pulls the hot water one way or otherwise it goes back to the wood boiler. Normally the water circulates to the top of the oil boiler and to the bottom of the wood boiler. The pipe that discharges out of the boiler goes to three way motorized valve that fails in one position. there is circ pump that pumps water from the top of the boiler to the bottom, the pump is off until a thermoswitch mounted hald way up the water jacket gets to 140 degrees. Once the storage is over 140 F the oil burner is locked out with fail close relay. The boiler is in series with an oil boiler, if the storage is below 140 degrees the oil boiler takes over like the wood boiler isnt there. I am short on radiators so wen its really cold 140 F water really doesnt cut it so I tend to fire it more often by shorter runs. I get around 24 hours between firings, double that during the cold windy weather of the last two weeks. The boiler itself probably has 80 gallons of storage plus 1000 pounds of mass so when its heated up, it releases heat slowly to the basement and up through the main floor. Therefore I can store about (185-140)*8#s/gallon*500 gallons = 180,000 btus which is the equivalent of 2 gallons of #2 fuel oil fied at 87% efficiency. I have 550 gallons of pressurized storage good for 185 degs max. If the storage is over 140F, the oil boiler burner is locked out but the circulator and zone controls are working using water from storage. My wood boiler is in series with an oil boiler. I have 500 gallons of unpressurized storage. It was coal/wood rated for 85,000 btu/hr input. Its an ASME coded boiler and was built beefy. It has half burnt off turbulators down the tubes. Above the baffles are a lower tube sheet with 16 or 20 tubes that then empty into an upper ash chamber before it vents out the side. My configuration is a bottom grate feeding into triangular refractory pit where the wood goes and then water jacketed fire box with baffle plates about a foot above the refractory pit. I have the same damper setup, its either full on or full off. I inspect the chimney every year and run a brush out of guilt every 5 years or so.
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